Sensory information continuously floods our perceptual systems. Success at attending selectively to a particular environmental stimulus requires us to ignore competing stimuli. This project examines the brain mechanisms that underlie perceptual processing of competing auditory distractors. Cognitive science has long debated the extent to which irrelevant stimuli are processed, with theorists since the 1950s advocating either early selection (gating at the sensory level leading to limited processing of distractors) or late selection (gating at the awareness level after complete sensory processing of distractors). The two views are reconciled in a recent model (Lavie, 1995, 2000;Lavie et al., 2004) that determines the locus of selection according to the load level (high, low) and the mental processing (perceptual, cognitive control) demanded by the attention task. According to this "load" model, successful selective attention is best achieved under conditions of high perceptual load or low cognitive control load. Considerable behavioral evidence and some neuroimaging data lend support to the load model, at least in the visual modality. Corresponding studies in the auditory modality are scarce and involve mainly behavioral and electrophysiological measures. We propose to investigate, using simultaneous recordings of ERPs and fMRI, how perceptual load and cognitive control load (i.e., working memory) each influence the perceptual processing of task-irrelevant speech sounds along the auditory pathway. Speech sounds are an ideal choice as distractors because they are highly relevant to humans, and their processing has been relatively well localized in the brain. In addition, complex acoustically-matched non- speech sounds will be employed to test the specificity of selective attention effects to phonemic perception. We propose to provide direct neural evidence bearing on the debate between early and late selection views in the auditory modality. Specific Aim 1 is to study the extent to which the processing of irrelevant speech sounds in the cortical auditory system is modulated by the perceptual demands of a primary task. In accordance with the load model, we hypothesize a reduction in brain activity in speech-related areas and smaller distractor-related ERP responses under conditions of high perceptual load. Specific Aim 2 is to study the extent to which the processing of irrelevant speech sounds in the cortical auditory system is modulated by the cognitive control demanded by the primary task. In accordance with the load model, we hypothesize a reduction in brain activity in speech-related areas and smaller distractor-related ERP responses under conditions of low working memory load. This project will provide some of the first neuroimaging evidence concerning cortical mechanisms of selective attention in the healthy human auditory system. The results should provide a relatively unambiguous test of the load model of selective attention. Project Narrative: In addition to more clearly delineating the neuroanatomic foundations of our auditory perceptual and attention system, the findings will help identify and decipher the processing deficits that underlie auditory attention impairments in schizophrenia and dyslexia, which will be invaluable for formulating effective approaches to remediation in these conditions.